GMC SIERRA DENALI 2002 Manual page 162

(Floor): This mode directs most of the air through
the floor outlets and center floor console outlets, as well
as some air through the windshield defroster outlets and
side window defroster outlets. If air from the center
floor console outlets is not desired, turn the vents off at
the console.
(Defog): Airflow is delivered through the floor
outlets, center floor console outlets, the windshield
defroster outlets and the side window defroster outlets.
The air flow can be divided between floor and defrost
depending upon where the knob is placed. When in
blend mode, the air conditioning compressor may run
to assist in dehumidifying the air and limit window
fogging.
(Defrost): This mode directs most of the air
through the windshield defroster outlets with airflow
also delivered from the side window and the floor
outlets. When in defrost mode, the air conditioning
compressor may run to assist in dehumidifying the air
and limit window fogging.
Mode Buttons
(Outside Air): This mode should be used for
normal system operation and directs the system to use
outside air. Using outside air will limit stale air odors
and help to prevent excessively humid interior air
(from wet boots or clothing or maximum passenger
loads) from being redirected onto windows.
Pressing the outside air button will cancel the
recirculation button.
(Recirculation): This mode limits the amount of
outside air entering the vehicle to a very small amount.
This will be helpful to limit odors entering the vehicle
and will assist in reaching comfort in very hot
temperature conditions. Pressing recirculation will cause
the system to recirculate interior air continuously and
may cause the vehicle windows to fog if the passenger
compartment floor is excessively wet.
Pressing the recirculation button will cancel outside air
operation. To return to normal system operation and to
prevent stale passenger compartment air, be sure to
return to the outside air position.
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